(1) Technical Field
The present invention relates to a semiconductor laser module, a Raman amplifier, and a method of manufacturing the semiconductor laser module, and more specifically to a semiconductor laser module in which laser light emitted from a semiconductor laser device is coupled to an optical fiber, a Raman amplifier, and a method of manufacturing the semiconductor laser module.
(2) Description of Related Art
With progress in the optical communications based on a dense wavelength division multiplexing transmission system over the recent years, a higher output is increasingly demanded to a pumping light source used for the optical amplifier.
Further, a greater expectation is recently given to a Raman amplifier as an amplifier for amplifying the light having a much broader band than by an EDFA (erbium-doped optical fiber amplifier) that has hitherto been employed as the optical amplifier. The Raman amplifier may be defined as a method of amplifying the optical signals, which utilizes such a phenomenon that a gain occurs on the side of frequencies as low as 13 THz on the basis of a pumping wavelength due to the stimulated Raman scattering occurred when the pumping light enter an optical fiber, and, when the signal beams having the wavelength range containing the gain described above are inputted to the optical fiber in the thus excited state, these signals are amplified.
According to the Raman amplification, the signal beams are amplified in a state where a polarization direction of the signal beams is coincident with a polarization direction of the pumping lights, and it is therefore required that an influence by a deviation between polarization directions of the signal beams and of the pumping lights be minimized. For attaining this, a degree of polarization (DOP) has hitherto been reduced by obviating the polarization of the pumping lights, which may be called depolarization.
A conventionally known technique for reducing the degree of polarization of a pumping light source is to couple a depolarizer (PMF: polarization maintaining fiber) to the tip of an output optical fiber that outputs laser light from a semiconductor laser module to the outside, and to arrange the polarization maintaining fiber with its principal axis tilted at an angle of 45 degrees with respect to the laser polarization direction (hereinafter the technique is referred to as Prior Art 1).
As a method for obtaining an increased optical output from a pumping light source and simultaneously depolarizing a laser beam emitted therefrom, one in which two laser beams emitted from two semiconductor laser devices oscillating at identical wavelengths are polarization-synthesized by use of a polarization-synthesizing coupler is known, as disclosed in U.S. Pat. No. 5,589,684.
FIG. 28 is an explanatory diagram showing a conventional semiconductor laser apparatus as disclosed in U.S. Pat. No. 5,589,684.
As shown in FIG. 28, the conventional semiconductor laser apparatus comprises a first semiconductor laser device 100 and a second semiconductor laser device 101 emitting laser beams of identical wavelengths in mutually orthogonal directions; a first collimating lens 102 configured to collimate the laser beam emitted from the first semiconductor laser device 100; a second collimating lens 103 configured to collimate the laser beam emitted from the second semiconductor laser device 101; a polarization-synthesizing coupler (i.e. cube beam splitter) 104 configured to polarization-synthesize the laser beams that were collimated by the first collimating lens 102 and the second collimating lens 103; a convergent lens 105 configured to converge the laser beams polarization-synthesized by the polarization-synthesizing coupler 104; and an optical fiber 106 for receiving the laser beams converged by the convergent lens 105 and letting the laser beams travel outside.
In the prior art, the laser beams are emitted from the first semiconductor laser device 100 and the second semiconductor laser device 101 in mutually orthogonal directions and are polarization-synthesized by the polarization-synthesizing coupler 104 to obtain a laser beam of reduced DOP from the optical fiber 106 (hereinafter the technique is referred to as Prior Art 2).
In addition, the applicant of the present invention has proposed a semiconductor laser module in which two laser beams emitted from a single semiconductor laser device having two stripes are polarization-synthesized and received by an optical fiber. (See Japanese patent application No. 2001-402819, for example. This technology will hereinafter be called a related art.)
FIG. 29 is an explanatory diagram schematically showing a configuration of the semiconductor laser module of the related art.
As shown in FIG. 29, the semiconductor laser module M12 of the related art includes a single semiconductor laser device 2 having a first stripe (a light-emitting stripe) 9 and a second stripe 10 formed with a spacing interposed therebetween and emitting a first laser beam K1 and a second laser beam K2 from a front end face (i.e. an end face on right-hand side in FIG. 29) of the first stripe 9 and the second stripe 10 respectively; a first lens 4 configured such that the first laser beam K1 and the second laser beam K2 are incident therealong and separated in the direction in which the first and second stripes 9, 10 are arrayed; a half-wave plate 6 configured to rotate a polarization direction of at least one of the first and second laser beam K1, K2 (i.e. the first laser beam K1 in FIG. 29) by a predetermined angle (by 90 degrees, for example); a PBC (polarization beam combiner) 7 configured to optically synthesize therealong the first laser beam K1 and the second laser beam K2; and an optical fiber 8 for receiving the laser beams emerging from the PBC 7 and letting the beams to travel outside.
In addition, a prism 5 is disposed between the first lens 4 and the half-wave plate 6 so that the first laser beam K1 and the second laser beam K2 are incident thereon and output therefrom along their respective optical axes parallel to each other. Further, a second lens 16 is disposed between the PBC 7 and the optical fiber 8 in order to optically couple the first and second laser beams K1, K2 polarization-synthesized by the PBC 7 to the optical fiber 8.
The first laser beam K1 and the second laser beam K2 emitted respectively from the front end face 2a of the first stripe 9 and the second stripe 10 of the semiconductor laser device 2 travel through the first lens 4, intersect and separate until the separation between the two beams is enough, before entering the prism 5.
The first laser beam K1 and the second laser beam K2 are made parallel to each other during propagation through the prism 5, and are output from there. The first laser beam K1 then enters the half-wave plate 6, where its polarization direction is rotated by 90 degrees, and then enters a first input port 7a of the PBC 7, while the second laser beam K2 enters a second input port 7b of the PBC 7.
The first laser beam K1 incident on the first input port 7a and the second laser beam K2 incident on the second input port 7b are synthesized along the PBC 7, and output from an output port 7c. 
The laser beams emerging from the PBC 7 are then converged by the second lens 16, enter an end face of the optical fiber 8 supported by the ferrule 23, and propagate to outside.
In Prior Art 1, when the polarization maintaining fiber that serves as a depolarizer is fusion-spliced to the end of the output optical fiber, it is difficult to position with accuracy the polarization maintaining fiber so that the principal axis of the polarization maintaining fiber tilts at an angle of 45 degrees with respect to the direction in which the laser light is polarized. Another problem of Prior Art 1 is high cost because a long polarization maintaining fiber has to be used in order to lower the degree of polarization satisfactorily.
In Prior Art 2, the first semiconductor laser device 100 and the second semiconductor laser device 200 are different from each other in terms of light intensity and efficiency in coupling with the optical fiber. For that and other reasons, Prior Art 2 in some cases fails to satisfactorily lower the degree of polarization (DOP) of laser light that is optically coupled to the optical fiber 106 through polarization synthesis.
In the semiconductor laser module according to the related art, the amount of light coupled to the optical fiber 8 differs from the laser beam K1 to the laser beam K2 because of the following reasons, for example, (1) the laser beam K1 alone passes through the half-wave plate 6, and the length of light path to the optical fiber 8 is not equal for the laser beam K1 and K2, which means that the loss the laser beam K1 suffers along the propagation is different from the loss the laser beam K2 suffers along the propagation, (2) the difference in optical length of the light path to the optical fiber 8 between the laser beam K1 and K2 causes a difference in position of the beam waist between laser beam that is formed after K1 exits the second lens 16 and laser beam that is formed after K2 exits the second lens 16, and (3) the laser beam K1 and K2 emitted from the first and second stripes, are different from each other in intensity, spreading angle, and others. The difference in beam amount due to those and other reasons sometimes causes the semiconductor laser module to fail in satisfactorily lowering the degree of polarization (DOP) of laser light coupled to the optical fiber 8.
Furthermore, the long-term use may degrade one of the stripes and change the position of the members relative to one another, which could lead to an increase in degree of polarization.
In addition, in Prior Art 2 and the semiconductor laser module according to related art, when efficiency in coupling each laser light to the optical fiber is changed due to a change in environmental temperature or the like, degree of polarization of laser light outputted from the optical fiber could fluctuate.